Capillary electrophoresis (CE) is a powerful analytical separation technique for the analysis of complex mixtures. In CE, an unknown sample is introduced at an Inlet of a capillary channel filled with a buffer solution, and a high voltage is applied across a section of the capillary. Different constituents of the sample migrate through the capillary at different rates depending on their electrophoretic mobility's, and are separated into different zones. By detecting the chemicals passing through a part of the capillary or its outlet as a function of time, and knowing the of the possible constituents, the chemical composition of the sample can be determined. A number of detectors have been developed for CE, including optical and electrochemical methods. Electrochemical detection can be classified into three main categories: amperometry, voltammetry and conductivity measurements. Conductivity detection is a non-selective detection mode and universally applicable. Analytes are detected because of their different conductivities to that of the background electrolyte.
One method to measure conductivity during electrophoresis is potential gradient detection, which is accomplished by putting two electrodes in the applied electric field for electrophoresis and detecting sample components by measuring potential changes between these two electrodes when sample components are passing by. This method has been used for isotachophoresis (U.S. Pat. No. 3,941,678, 20 Feb. 1975 and U.S. Pat. No. 3,932,264, 13 Jan. 1976) and it has been mentioned that such a method can be used in modem CE (F. Foret, L. Krivankova and P. Bocek, Capillary Zone Electrophoresis, chapter 7, p147-150). There are, however, problems for using this method in electrophoresis. Firstly, the sensing electrodes need to be inserted into the separation column or capillary. The procedures are troublesome and tedious, especially if the inner diameter of the capillary is small, for example in the case of capillary electrophoresis (usually between 10-100 μm). The more serious problem is that the sensing electrodes are polarized during electrophoresis. In order to prevent formation of bubbles and deposits on the electrodes so that the electrophoresis processes can be performed under stable conditions and high sensitivity can be obtained, special means have to be used, such as adopting v/F and F/v converters in the instrumental design, reducing the areas of electrodes contacting with the buffer solutions and adding nonionic surfactant. However, all these means can only serve to alleviate, but can't eliminate completely the problems encountered.
Therefore, conductivity detection is usually accomplished by measuring the potential difference (signal) between two electrodes while passing through a small constant current (excited source). Several designs are used for conductivity detection in CE, i.e., on-column, end-column and contactless structures. On-column detection cells (Anal. Chem., 1987, 59, 2747-2749, U.S. Pat. No. 5,223,114, 29 Jun. 1993 and U.S. Pat. No. 5,580,435, 1994) are usually made by inserting two sensing platinum wires into the separation capillary so that the sensing electrodes can directly contact the electrolyte solution in the capillary. Although on-column conductometric detection works well, the question arises as to how to produce such structures reliably and inexpensively. The more common practices are the use of end-column detectors (such as those disclosed in Anal. Chem. 1991, 63, 189-192, J. of Capillary Electrophoresis, 1996, 1:1-11, U.S. Pat. Nos. 5,298,139, and 5,126,023), which have the advantage that the sensing electrode can be constructed directly at the outlet of the separation capillary. For end-column detection, the correct alignment of the sensing electrode with the outlet of the separation capillary is critical for success. However, the alignment is usually difficult due to the small inner diameter (10 μm-100 μm) of the capillary.
Another solution offered in the prior art is contactless conductivity detection (Anal. Chem., 1998, 70, 563-567). In this method, two electrodes are laid on the outside wall of the separation capillary. Therefore, no electrode is in contact with electrolyte solution. However, it is generally accepted that the contactless detection is not sensitive enough. Although their structures are varied, all the prior designs should use their own excited source and considered the high voltage applied for electrophoresis as a noise source.
Although the three techniques described above (i.e. potential gradient detection, potential difference detection, and contactless conductivity detection) are all based on the difference in conductivity between the electrophoretic medium and the samples, potential difference detection is the most widely used technique in capillary electrophoresis. Therefore, commercially available and commonly described conductivity detection systems typically employ the potential difference detection method.